On 8/3/07, Chris Smith <cdsmith at twu.net> wrote:
> Yes, unless of course you did:
>> instance (Monad m, Num n) => Num (m n)
>> or some such nonsense. :)
I decided to take this as a dare - at first I thought it would be easy
to declare (Monad m, Num n) => m n to be an instance of Num (just lift
or return the operators as necessary), but I ran into trouble once I
realized I needed two things I wasn't going to get:
An instance of Eq (m n), and an instance of Show (m n) for all monads
m. Eq would need a function of the form:
(==) :: Monad m => m a -> m a -> Bool
and Show would need a function of type m a -> String
There's no way I'm getting a function of those types using return and
join to operate on the monad.
So, there went that idea.
-Antoine